Our viability and cytotoxicity assay reagents (Viability and Cytotoxicity Assay Reagents—Section 15.2) and kits (Viability and Cytotoxicity Assay Kits for Diverse Cell Types—Section 15.3) are principally used to enumerate the proportion of live and dead cells in a population. In contrast, proliferation assays such as the Click-iT EdU cell proliferation assay (Assays for Cell Enumeration, Cell Proliferation and Cell Cycle—Section 15.4) are primarily designed to monitor the growth rate of a cell population or to detect daughter cells in a growing population. Fluorescence-based cell viability and proliferation assays are generally less hazardous and less expensive than radioisotopic techniques, more sensitive than colorimetric methods and more convenient than animal testing methods. Unlike 51Cr-release assays, fluorescence-based assays of cell-mediated cytotoxicity do not require large samples, which can be difficult to obtain. Furthermore, fluorescence-based protocols are more convenient than the trypan blue–based exclusion assay. This common colorimetric method for determining cell viability must be completed within 3–5 minutes because the number of blue-staining cells increases with time after addition of the dye.

In addition to our probes for cell viability, cell proliferation, apoptosis and autophagy, several of the reagents for live-cell function described in Probes for Cell Adhesion, Chemotaxis, Multidrug Resistance and Glutathione—Section 15.6 can be used to develop assays that measure a particular biochemical parameter of interest. There is a significant overlap between probes for cell viability and probes for live-cell functions. For example, fluorogenic esterase substrates are commonly used to detect viability and proliferation, as well as to monitor cell adhesion, apoptosis and multidrug resistance. Likewise, cell-permeant and cell-impermeant nucleic acid stains are widely applicable to many live-cell function assays. We have organized discussions in this chapter according to several commonly studied cell processes in order to highlight the many published applications for these probes and foster the development of new applications.

The diversity of live cells and their environments () makes it impossible to devise a single viability or enumeration assay applicable to all cell types. Because viability is not easily defined in terms of a single physiological or morphological parameter, it is often desirable to combine several different measures, such as enzymatic activity, membrane permeability and oxidation–reduction (redox) potential. Each assay method has inherent advantages and limitations and may introduce specific biases into the experiment; thus, different applications often call for different approaches.